In general, a refrigerator is constituted by a compressor, a condenser, an expansion mechanism (such as, an expansion valve), and an evaporator, and further by a dryer, and has a structure, in which a mixture of a refrigerant and a refrigerator oil is circulated in a closed system. In a refrigerator, the mixed liquid of a refrigerant and a lubricating oil is used in a wide temperature range of from a low temperature to a high temperature since a high temperature occurs in a compressor, whereas a low temperature occurs in an evaporator and the like.
As a refrigerant used in a refrigerator in recent years, a refrigerant containing no chlorine is used as a refrigerant that does not deplete the ozone layer, and examples thereof used include fluorinated hydrocarbon refrigerants represented by 1,1,1,2-tetrafluoroethane (R134a), difluoromethane (R32), pentafluoroethane (R125), and 1,1,1-trifluoroethane (R143a).
As a refrigerant that has a low global warming potential, among the fluorinated hydrocarbon refrigerants, an unsaturated fluorinated hydrocarbon refrigerant, such as 1,3,3,3-tetrafluoropropene (HFO1234ze) and 2,3,3,3-tetrafluoropropene (HFO1234yf), are being used, for example, in a car air-conditioner and the like. Furthermore, carbon dioxide may be used in some cases for some purposes including a hot water supplier or the like since carbon dioxide is harmless to the environments and the human bodies, is easily available, and is unnecessary to recover.
The refrigerator oils vary in demanded characteristics depending on the refrigerant used, and base oils and additives added to the base oils have been developed corresponding to the refrigerants. For example, an unsaturated fluorinated hydrocarbon refrigerant may form a wax component in some cases through polymerization of the refrigerant and the base oil under a high-speed and high-load condition due to the low heat stability and the low chemical stability thereof. Accordingly, for example, PTLs 1 to 4 have studied the addition of a polymer paraffin inhibitor, such as ethylene glycol, for preventing the decomposition of the unsaturated fluorinated hydrocarbon refrigerant and the formation of a polymer paraffin due to the polymerization of the polymerizable decomposition product.